Field investigation of three-dimensional flow structure at stream confluences 2. Turbulence

Stream confluences are among the most highly turbulent locations in fluvial systems. This paper examines the three-dimensional structure of turbulence at three stream confluences in east central Illinois. The analysis focuses on the characteristics of turbulence both within the shear layer and in the ambient flow. Results show that at the upstream end of each confluence the shear layer occupies a limited portion of the flow cross-sectional area, but turbulence kinetic energy within this layer is 2-3 times greater than the turbulence kinetic energy of the ambient flow, which has turbulence characteristics similar to those for flow in straight channels. Turbulence within the shear layer can be characterized as quasi-two-dimensional in the sense that large-scale turbulence generated by transverse shear is predominantly two dimensional, whereas small-scale turbulence associated with bed friction is three dimensional. Spectral analysis suggests that the structure of fluid motion within the shear layer differs for confluences with symmetrical versus asymmetrical planforms. The shear layer dissipates rapidly as flow enters the downstream channel, even though a well-defined mixing interface persists at downstream locations.